The present invention relates in general to medical devices and techniques, and in particular to measurement of physiological parameters using dependence of blood resistivity on flow.
It has long been known that the resistivity of blood varies directly with the flow velocity and inversely with hematocrit, i.e., the percentage, by volume, of whole blood that is made up of red blood cells. (See, e.g., Hoetnik et al., IEEE Trans. Biomed. Engr. 51:7, 1251 (2004); Sigman et al., Amer. J. Physiol., 118, 708 (1937)). The variation can be on the order of 15%, large enough to be measurable. It is generally believed that the resistivity varies due to the difference in conductivity between red blood cells (RBCs) and plasma. As blood flows, the RBCs order and deform, which disrupts currents flowing through the blood, resulting in variation in the bulk resistance of blood.
Blood flow velocity can provide an indication of a patient's physiological functioning. For instance, blood flow velocity near the heart can indicate whether the heart is operating effectively. Vessel blockages created by blood clots, plaque deposits, or the like also affect blood flow velocity, as do internal bleeding and other conditions.
Existing blood flow measurement systems exploit this property, but they are often limited to use in a clinical setting. For instance, measurements can be made by inserting appropriate sensors during catheterization and other invasive procedures, but the sensors are not designed to be left in place after such procedures are completed; consequently, data cannot be collected while the patient engages in normal daily activity outside the clinical setting. Thus, the ability to use information about blood flow to monitor or diagnose disease conditions is limited.
It would therefore be desirable to provide improved blood flow sensors capable of being used for ongoing monitoring of a patient's condition.